In general, the electric field operator, considering only one polarisation direction, can be written as [1]}:
\(\begin{split}&E(r,t)=E^{(+)}(r,t)+E^{(-)}(r,t)=\\&=\sum _{k}e_{k}E_{k}a_{k}e^{-i(\omega _{k}t-kr)}+h.c.\end{split}\) 
